It is known that improving fuel efficiency is an ongoing goal for both government and industry, both within the United States and internationally. Fuel costs have historically been the largest single cost associated with aircraft operations; improved efficiency therefore translates directly to the bottom line. The worldwide aviation industry is a significant emitter of carbon dioxide and other greenhouse gases; the International Civil Aviation Organization (ICAO) puts it at 2% of the global anthropogenic total. The impact of these emissions is amplified even more, however, because they go directly into the upper troposphere. Both Government regulators and industry associations have set aggressive goals for reducing these emissions, but these will require significant new technology. An effective drag-reducing technique will directly assist in reducing fuel consumption, and hence reduce fuel expenses and greenhouse gas emissions.
It is well known that streamwise vorticity dominates near-wall turbulence production and skin friction drag. As such, efforts to intervene in the self-sustaining mechanism of streamwise vortex formation and instability will yield drag reduction. Prior research described a new mechanism for coherent structure generation in the self-sustaining mechanism of near-wall turbulence. Their results strongly suggest that normal mode low-speed streak instability is not a significant contributor to streamwise vortex growth and near-wall turbulence production. Rather, they proposed and demonstrated a new “Streak Transient Growth Instability” (STGI) as the dominant streamwise vortex generation mechanism. Their work showed that STGI can produce order-of-magnitude linear growth of streamwise disturbances.
Other works, based on direct numerical simulations of channel flow, have proposed a strategy for drag reduction by actively intervening in the STGI process. In particular, they found that streamwise coherent structures in near-wall turbulence are created by the sinuous instability of lifted vortex-free streaks due to the presence of previous vortices. They proposed a method of large-scale flow control for drag reduction, which exploits the fact that the low-speed streak growth rate depends critically on the wall-normal vorticity, ωy, flanking the streak as shown in FIG. 1. This figure presents the instability growth rate as a function of wall normal vorticity and clearly delineates regions of stability and instability based on the magnitude of ωy.
The authors demonstrate that control schemes based on decreasing ωy are successful in achieving very significant drag reduction (e.g. up to 50% in their channel flow DNS). They found that control in the form of either spanwise colliding wall jets or an array of 2D counter-rotating vortices was able to break the cycle of near-wall vortex generation by disrupting the unstable streak distribution due to older, preexisting streamwise vortices. Their approach has the advantage of achieving distributed flow control without the need for any flow sensors or supporting control logic.